1. Field of the Invention
The present invention concerns a semiconductor device having a circuit constituted with thin film transistors (hereinafter referred to as a TFT). It relates to a constitution of an electro-optic device typically represented by a liquid crystal display panel, and an electronic equipment having such an electro-optic device mounted as a part thereon. The semiconductor device used in the present specification generally means all devices that can operate by utilizing semiconductor characteristics, and electro-optical devices, semiconductor circuits and electronic equipments are also included in the semiconductor device.
2. Description of the Related Art
In recent years, an active matrix type liquid crystal display device having circuits constituted with TFTs utilizing polysilicon films has been noted. In the device, electric fields applied to liquid crystals are controlled in a matrix form by a plurality of pixels disposed in a matrix to attain highly fine image display.
The active matrix type liquid crystal display device described above has pixels by the number of 1,000,000 or more as the resolution becomes higher as an XGA and an SXGA. Then, driver circuits for driving all of them is extremely complicated and formed with many TFTs.
Since the specification required for the actual liquid crystal display device (also referred to as liquid crystal panel) is stringent and it is necessary to ensure high reliability both for pixels and drivers in order to operate all the pixels normally. Particularly, if abnormality should occur in the driver circuit, a failure referred to as a line defect in which all pixels in one row (or in one column) are put to disorder.
By the way, it is considered that a TFT utilizing a polysilicon film has not yet been comparable with an MOSFET (transistor formed on a single crystal semiconductor substrate) used for LSIs in view of the reliability, and it has become considered that an LSI circuit can not be formed easily with TFTs unless the drawback point described above can be overcome.
The present applicant considers that the MOSFET has three advantageous features in view of the reliability and assumes the reason as below. FIG. 2A is a schematic view of an MOSFET, in which are shown a drain region 201 formed on a single crystal silicon substrate, an LDD (Lightly Doped Drain) region 202, a field insulation film 203 and a gate insulation film 205 just below a gate wiring 204.
This constitution provides three advantageous features in view of the reliability. The first advantageous feature is presence of a gradient in the impurity concentration from the LDD region 202 to the drain region 201. As shown in FIG. 2B, the impurity concentration increases gradually from the LDD region 202 to the drain region 201 in the existent MOSFET. It is considered that the gradient has an effect of improving the reliability.
The second advantageous feature is that the LDD region 202 and the gate wiring 204 overlap with each other. As the structure, a GOLD (gate-drain overlapped LDD) or an LATID (large-tilt-angle implanted drain) is known. This constitution can decrease the impurity concentration in the LDD region 202 and an electric field moderating effect is increased to improve the hot carrier resistance.
The third advantageous feature is that there is a certain distance between the LDD region 202 and the gate wiring 204. This is because the field insulation film 203 is formed in a state creeping just below the gate wiring. That is, since the thickness of the gate insulation film is increased only in the overlapped portion, an effective electric field moderation can be expected.
As described above, the existent MOSFET has several advantageous features over the TFT and, as a result, has high reliability.
Furthermore, it has also been attempted to apply the advantage of the MOSFET described above to the TFT. For example, the GOLD structure is attained by using a side wall formed with silicon in “M. Hatano, H. Akimoto, and T. Sakai, IEDM 97 TECHNICAL DIGEST, pp. 523-526, 1997”.
However, the structure disclosed in this article has a problem that an off current (current flowing in an off state of a TFT) increases compared with the usual LDD structure, which necessitates a countermeasure therefor.
As described above, the present applicant considers that structural problems in the TFT give an undesired effect on the reliability (particularly, the hot carrier resistance) when comparing the TFT with the MOSFET.